Conventionally, milled toilet soaps are made by a process which comprises (1) drying soap having a moisture content of from about 28% to about 30% down to a moisture content of about 7% to about 14%, (2) forming the dried soap into noodles by passing it through a plodder, (3) mixing the various desired additives such as colorants, perfume, etc., into the soap noodles, (4) passing the mixture formed in (3) through a mill or series of mills ("milling" the soap) thereby forming ribbons of soap, (5) passing the milled soap mixture from (5) through a plodder to form a log of soap (i.e., "plodding" the soap), and (6) cutting the log into segments and stamping the segments into the desired bar shape.
The soap which is dried in step (1) can be made from saponification of fats or neutralization of free fatty acids. Because the drying is never completely uniform, the dried soap inevitably contains some particles which are overdried and are harder than the remaining bulk of the dried soap. If the soap also contains free fatty acid, nonhomogeniety of the free acid in the soap can also contribute to the presence of soap particles which are harder than the remaining bulk of the dried soap. The hard particles are generally from about 0.5 to about 10 mm in diameter. These particles remain in the soap through the first plodding step (2) and the mixing step (3). In the milling step (4), the soap is "worked" and the overdried particles are broken down into much smaller particles (generally less than about 0.25 mm in diameter) and are homogeneously distributed throughout the soap mass. In the absence of milling, the finished bar will exhibit a rough or sandy feel during use, due to the slower dissolution rate of the relatively large overdried soap particles, also called "hard specks." When the soap has been properly milled, the overdried soap cannot be detected during use, because it has been reduced to a much smaller particle size and is distributed uniformly throughout the soap mass. See British Pat. No. 512,551, Cruikshank, Sept. 19, 1939, incorporated herein by reference.
Glycerin is a desirable skin conditioning additive for soap bars. It can also be used to impart translucency to the soap. The present inventors have found that when high levels of glycerin (i.e., 2% to 25% of the finished bar) are added to the soap at the conventional place for introducing additives (i.e., in the mixing step prior to milling) the efficiency of the milling process in regard to the breakup and homogeneous distribution of overdried soap particles is greatly reduced. This, in turn, results in finished bars with a high incidence of detectable hard specks. It is believed that the glycerin "lubricates" the overdried soap particles thereby retarding breakup of said particles during the working of the soap which takes place during milling.
Since glycerinated soap is generally softer and more soluble than conventional soap, the presence of hard specks in a glycerinated soap matrix is even more noticeable to the touch than if they are present in a conventional soap bar.
The object of the present invention is to provide a process for making milled toilet bars containing a high level of glycerin and which are substantially free of hard specks.